Fairford Electronics QFE Series, Manual

Page: 54 / 194

Applications
QFE
page 3-6
FD841405_F
3.1.4 Maximum Motor Cable Length
The length of the cable between the output terminals of the starter and the motor should not
normally be greater than 100 metres.
3.1.5 Power Factor Correction Capacitors
Power factor correction capacitors applied to a single motor MUST always be connected by
a separate contactor placed on the SUPPLY side of the QFE soft-start. Capacitors should be
switched in after top-of-ramp (full line voltage) is reached and switched out of circuit before a
stop is initiated.
It is important that any total system PFC scheme that automatically corrects for a range of
inductive loads is not operated in such a way as to leave it heavily over compensated since
this might introduce oscillations leading to damaging over-voltages.
3.1.6 Lightly Loaded, Small Motors
Lightly loaded, small-sized (less than 2kW), star connected motors can produce high voltages
at the motor terminals when shut down by simply opening the line contactor. As these
voltages can damage the soft-starter, it is safer to control the opening of the line contactor
with the soft start run relay contacts.
3.1.7 Motors Fitted with Integral Brakes
Motors that include an integral, electrically operated brake, internally connected to the motor
input terminals, can only be soft-started when the brake is re-connected to the supply through
its own contactor.
3.1.8 Older Motors
The action of the fully-controlled soft-starter introduces harmonic currents and voltages to
the motor. It is therefore, important to ensure that the motor employs techniques such as
rotor skewing in its construction to suppress the effects of harmonic fluxes and avoid rough
starting. This is rarely a problem with modern motors because nearly all motors designed in
the last 20 years employ these techniques.
3.1.9 Wound-rotor or Slip-ring Motors
Slip-ring induction motors ALWAYS need some resistance in the rotor circuit to ensure that
sufficient rotational torque is generated to overcome any alignment torque, which is present
at start-up. The resistance can be safely shorted out in the normal fashion with a contactor
controlled by the programmable relay set as ‘top-of-ramp’ contacts.
3.1.10 Enclosures
Thyristors are not perfect conductors, and the passage of current through them causes heat
dissipation in the body of the device, which in turn causes the heatsink temperature to increase.
As a rough guide, the heat generated is 1 watt/amp/phase, which equates to a dissipation of
30 watts from the heatsink for a line current of 10 amps. Therefore, all cabinets or enclosures
that house soft-starters should have adequate ventilation. (Refer to the Mechanical installation
procedures, section 1.6 to 1.8 for more detailed information.)
3.1.11 Efficiency
Although the use of the soft-start introduces a power loss, the system still retains an overall
efficiency of approx. 99.5%. If the optimising function is selected, then the gain in motor
efficiency at part loads is far greater than the loss of efficiency arising from thyristor heat
losses. If prolonged operation at full load is expected, the thyristor loss can be eliminated
(as in some matched motor/pump drives) by closing a bypass contactor around the soft-
starter. (This contactor is readily controlled by the programmable relay set as ‘top-of-ramp’
contacts as the circuit in section 2.4.1 shows.)